1. Field of the Invention
The present invention relates to a magnet roller, and relates to a magnet roller in which a plurality of magnet pieces are joined at joining faces.
2. Description of the Related Art
Conventionally, magnet rollers are employed in for example copiers of the electrophotographic type, facsimile machines and laser printers. An example of such a magnet roller is the type, called the xe2x80x9cjoined typexe2x80x9d, in which the magnetic field pattern is formed by joining a plurality of magnet pieces which are magnetized with their magnetic grains aligned unidirectionally.
Joined type magnet rollers are employed in developing devices for high picture quality, since they make possible the formation of a sharp magnetic field pattern. However, there are limits to the extent to which the magnetic flux density of the developing pole of the magnetic field pattern in such a magnet roller (hereinbelow called xe2x80x9cspecified magnetic polexe2x80x9d) can be raised, which made further improvement in picture quality difficult to achieve.
Accordingly, in Laid-open Japanese Patent Publication No. H11-65283 xe2x80x9cmagnet rollerxe2x80x9d, the present inventors proposed a method of considerably raising the magnetic flux density of the specified magnetic pole by constituting this specified magnetic pole of a combination of two magnet pieces. This made it possible to further improve picture quality.
However, in the magnet roller of the above publication, two magnet pieces are required in order to raise the magnetic flux density of a single specified magnetic pole. The number of magnet pieces therefore became large, making it difficult to raise productivity in respect of the magnet roller and making it difficult to lower production costs.
Recently, high magnetic force has come to be required even for poles other than the specified magnetic pole (developing pole), such as for example the pole for restricting the layer thickness of the developer; thus high magnetic force has come to be required in two or more magnetic poles in a single magnet roller. For example, if high magnetic force is required for two magnetic poles in a single magnet roller, with the magnet roller of the above publication, four magnet pieces would be necessary in order to provide magnetic poles of high magnetic force.
The number of magnet pieces was thereby further increased, further increasing the difficulty of improving productivity of the magnet roller, and making it more difficult to lower production costs.
The present invention was made in view of the above problems, an object thereof being to provide a magnet roller wherein picture quality can be further improved by raising the magnetic flux density of the specified magnetic pole and/or other magnetic poles, and wherein a magnetic pole pattern with improved magnetic flux density of this specified magnetic pole and/or other magnetic poles can be achieved with low cost.
In order to achieve the above object, according to the present invention, in a magnet roller according to the present invention, wherein a plurality of magnet pieces are mounted at the periphery of a shaft by joining at joining faces, peaks of magnetic poles are generated on the lines of extension of the joining faces by setting the directions of orientation magnetization of adjacent magnet pieces facing the joining faces, the respective joining faces of the plurality of magnet pieces being made to coincide with roller radial directions of this magnet roller.
With a magnet roller according to the present invention constructed in this way, the respective joining faces of the plurality of magnet pieces are made to coincide with roller radial directions and the orientation magnetization directions of adjacent magnet pieces are set facing the joining faces. Consequently, repulsive magnetic fields are generated at the joining faces, and the peaks of magnetic poles can be generated on the lines of extension of the joining faces.
By causing the magnetic poles to be formed by the repulsive magnetic fields at the joining faces, high magnetic force can be obtained at a plurality of magnetic poles (specified magnetic pole and other magnetic poles).
Also, the peaks of the magnetic poles are caused to be generated on the lines of extension of the joining faces by making the respective joining faces of the plurality of magnet pieces coincide with radial directions of the roller. Consequently, the number of magnet pieces can be made the same as the number of magnetic poles required or can be restricted to the number of magnetic poles required +1.
Consequently, the productivity of the magnet roller can be raised, and production costs can be lowered.
According to the present invention, the sum of the angles of the orientation magnetization directions of at least one set of the adjacent magnet pieces is set at 30xc2x0 to 140xc2x0.
By setting the sum of the angles of the orientation magnetization directions of at least one set of the adjacent magnet pieces at 30xc2x0 to 140xc2x0, it can be arranged for the repulsive magnetic field to be generated in the most efficient manner. The magnetic flux density of for example the specified magnetic pole can therefore be made sufficiently large.
According to the present invention, the orientation magnetization directions of at least one set of the adjacent magnet pieces are made to converge towards the outside of the joining face.
By making the orientation magnetization directions of at least one set of the adjacent magnet pieces converge towards the outside of the joining face, the magnetic path length is made longer, thereby increasing the coefficient of permeance and making it possible to generate a repulsive magnetic field in most efficient manner. The magnetic flux density of for example the specified magnetic pole can thereby be raised even further.